Intraocular Lens Power Calculation in Cases of Anterior Segment OCT-Detected Subclinical Posterior Keratoconus
Narrative Responses:
Purpose
To compare intraocular lens power calculation errors in cataract cases with subclinical posterior keratoconus using corneal refractive powers measured by different instruments.
Methods
Design: Retrospective case report.
Four eyes of 4 patients (ages 74.8±13.0) with subclinical posterior keratoconus detected by anterior segment optical coherence tomography (AS-OCT)(CASIA:TOMEY) were enrolled. Auto keratometer (TONOREFⅡ:NIDEK) and IOLMaster (Carl Zeiss Meditec) measured keratometric values. The AS-OCT measured real refractive corneal power (Real-K) which considered anterior and posterior corneal power and corneal thickness, and anterior to posterior corneal curvature ratio (A/P). Predicted refractive errors were calculated from each keratometric values based on postoperative subjective refractive errors. To calculate IOL power, SPK/T formula with optimized A constant was applied.
Results
The Real-K showed smallest value among other corneal refractive powers in all eyes. The maximum and minimum differences from other corneal refractive powers were 2.4D and 0.71D, respectively. The maximum and minimum preoperative A/P ratio calculated from the AS-OCT was 1.25 and 1.45, respectively. When IOL master measured keratometric values were applied for intraocular lens calculation, all 4 eyes became hyperopic, and the maximum and minimum postoperative refraction errors were 0.34D and 1.15D, respectively. When the Real-K was used, the maximum and minimum errors were +0.1D and -0.65D, respectively.
Conclusion
Refractive IOL power calculation errors in posterior keratoconus cases are attributed to significantly higher A/P ratios than Gullstrand Schematic eye or the average normal values (1.19). Therefore, real corneal power considering both anterior and posterior corneal curvatures should be applied for IOL power calculations in subclinical posterior keratoconus cases.